Generation and maintenance of abnormal fluid pressures beneath a ramping thrust sheet: Isotropic permeability experiments

Abstract

We have investigated the mechanisms responsible for the evolution of excess pore pressures within and beneath a ramping thrust sheet (i.e. fluid flow, porosity compression, and thermal expansion of water) and the sensitivity of pore pressure to a variety of physical parameters (e.g. permeability, thrust sheet velocity, heat flux). Coupled pore pressure and temperature equations were solved numerically in two dimensions using a generalized hydrostratigraphy of North American thrust belts. Because of the lack of either symmetry or a steady-state in this problem, both deposition and thrust loading stages were simulated. The dominant mechanisms controlling pore pressure evolution were fluid flow and compression of pore space by vertical loading; thermal expansion of the fluids was found to be insignificant in generating excess pore pressures at common thrust loading rates. The results indicate that it is possible to generate high pore pressure to lithostatic pressure ratios (λ) within thrust sheets by depositional loading prior to thrusting. High values of λ are generated and maintained during thrust loading for reasonable assumptions about the conditions thought to have existed in thrust belts. Values of λ were not constant throughout the model. The highest λ values tended to concentrate near the surface of the model and within and below the toe of the thrust sheet. The magnitude and distribution of excess pore pressures and λ values were found to be especially sensitive to variations in permeability. Excess pore pressure generation by compression exceeded pore pressure dissipation by fluid flow for permeabilities less than approximately 10 −16 m 2; permeabilities greater than approximately 10 −16m 2 produced hydrostatic pore pressure gradients. The models demonstrate that permeability inhomogeneity due to lithologic variations may exert a strong control on the magnitude and spatial distribution of excess pore pressures within thrust sheets. In addition, these models indicate that it is unlikely that fluid pressure is high everywhere in a moving thrust sheet.